1. Field of the Invention
The present invention relates to a method for repairing line disconnection of a display device and a display device capable of repairing line disconnection, which are particularly suitable for application to a liquid crystal display device.
2. Related Background Art
A liquid crystal display device is provided with a matrix of number of signal lines and pixel electrodes. As the number of signal lines is on the increase because of the recent trend of larger and higher resolution liquid crystal display devices, the problem of line disconnection can occur more often. Line disconnection occurs due to pinholes and dust in the manufacturing process. In the event of disconnection, a proper voltage is not applied to a pixel electrode corresponding to a disconnected line, causing line defects and other display problems to produce a defective product. Therefore, a method for repairing line disconnection using a laser is now under research and development.
FIG. 7 shows a configuration of a conventional active matrix liquid crystal display device. FIG. 7 is a plan view showing a structure of one pixel area of the liquid crystal display device. Reference numeral 12 designates a pixel electrode, 13 a thin film transistor (TFT), 15 a gate line, 16 a storage capacitor line, 17 a source line, and 28 a disconnected portion.
The active matrix liquid crystal display device shown in FIG. 7 is provided with a matrix of plurality of the pixel electrodes 12, each of which is connected with the TFT 13 that is a switching element. The gate electrode of the TFT 13 is connected with the gate line 15, and gate signals input to the gate electrode control and drive the TFT 13. The source electrode of the TFT 13 is connected with the source line 17, and data (display) signals are input to the pixel electrode 12 through the TFT 13 when the TFT 13 is selected. The gate line 15 and the source line 17 cross each other at right angles, surrounding the pixel electrode 12. The drain electrode of the TFT 13 is connected with the pixel electrode 12.
A method for repairing signal line disconnection is described in Japanese Patent Application Laid-Open No. H09-113930, for example. The method according to the first embodiment of the above prior art will be explained hereinbelow. FIG. 8A shows a structure of a cross-section of a disconnected portion of a gate line. The same reference numerals as those in FIG. 7 designate the same elements, and redundant description will be omitted. Reference numeral 11 designates a substrate, 22 a gate insulating film, and 29 a molten metal.
As shown in FIG. 7, the pixel electrode 12 is arranged to partially overlap the gate line 15. They are electrically isolated by the gate insulating film 22 to enlarge the area of the pixel electrode 12, that is, to increase the aperture ratio. Also, an additional capacitor is formed by placing the gate insulation film 22 between the pixel electrode 12 and the gate line 15.
As shown in FIG. 7, there are cross marks on both outsides of the disconnected portion 28 of the gate line 15. A laser is applied to the marks. The gate line 15 or the pixel electrode 12 is molten to produce the molten metal 29 as shown in FIG. 8B. A bypass route through the gate line 15, the molten metal 29, the pixel electrode 12, the molten metal 29, and the gate line 15 is thereby created; therefore, the disconnection is repaired. Disconnection of the source line 17 and the storage capacitor line 16 is also repaired in the same way.
FIGS. 9 and 10 show a structure of one pixel area of the liquid crystal display device where disconnection is repaired according to the second embodiment of the invention disclosed in Japanese Patent Application Laid-Open No. H09-113930. FIG. 9 is a plan view of a structure of one pixel area where disconnection is repaired, and FIG. 10 is a sectional view thereof. The same reference numerals as those in FIGS. 7 and 8 designate the same elements, and redundant description will be omitted. Reference numeral 41 designates a conductive layer for repairing gate line disconnection, 42 a conductive layer for repairing storage capacitor line disconnection, and 43 a conductive layer for repairing source line disconnection.
As shown in FIGS. 9 and 10A, the pixel electrode 12 partially overlaps the gate line 15 with the insulating film interposed therebetween. On the part of the pixel electrode 12 overlapping the gate line 15 is formed a conductive metal layer. The conductive metal layer provided for repairing disconnection of the gate line 15 will be referred to hereinafter as a conductive layer 41. Similarly, conductive metal layers for repairing disconnection of the source line 17 and the storage capacitor line 16 will be-referred to as a conductive layer 42 and a conductive layer 43, respectively.
In the following, a description will be given on the case where the gate line 15 is disconnected. As shown in FIGS. 9 and 10A, on the insulating substrate 11 are formed the gate line 15 and the storage capacitor line 16. The gate insulating film 22 is formed thereon. Next, the source line 17, the TFT 13, and an insulating layer are formed. Then, the pixel electrode 12 is formed. Further, the conductive layer 41 for repairing gate line disconnection is formed. The conductive layer 41 is provided on the area where the pixel electrode 12 overlaps the gate line 15 with the gate insulating film 22 interposed therebetween. The conductive layer 41 is formed in an island shape above the gate line 15 except the crossing with the source line 17.
Disconnection of the gate line 15, the storage capacitor line 16, and the source line 17 occurs due to pinholes and dust in the manufacturing process. If the disconnection occurs, no drive signal is given to the pixel, thus disabling display.
The case where disconnection occurs at the disconnected portion 28 of the gate line 15 of an active matrix liquid crystal display device will be explained hereinbelow. A laser is applied to the positions at both outsides of the disconnected portion 28 (the positions shown by the cross marks in FIG. 9) through the conductive layer 41. The molten metal 29 produced by the laser application electrically connects the conductive layer 41 and the gate line 15. The disconnected gate line 15 regains continuity by a bypass line through the molten metal 29, the conductive layer 41, and the molten metal 29. It is therefore possible to apply drive signals to the gate line where no signal has been given.
Disconnection of the source line 17 can be also repaired in the same way as shown in FIG. 11. Further, the storage capacitor line 16 can be repaired in the same manner.
The above conventional technique, however, has the following problems. In the first embodiment of the invention disclosed in Japanese Patent Application Laid-Open No. H09-113930, the connection resistance of the pixel electrode 12 and the molten metal 29 can be high. The pixel electrode 12 is formed by Indium Tin Oxide (ITO), and the connection resistance with chromium, tantalum, titanium, and molybdenum used for the gate line 15 and the storage capacitor line 16 significantly differs depending on conditions of film deposition and film surface. There is a case where the connection resistance reaches several megohms, which results in decrease in the rate of successful repair. Use of aluminum for the gate line 15 is especially problematic since the connection resistance of an aluminum element and ITO used for the pixel electrode 12 is extremely high. Therefore, when a laser is applied to connect them, the connection electrical resistance almost reaches one megohm, which is above the limit of the connection resistance for repairing disconnection. Besides, the application of a laser to the pixel electrode 12 causes the ITO to be peeled off and a fragment of the ITO is stuck in between a counter electrode and the pixel electrode to trigger unexpected short-circuit.
The repair method described in the second embodiment of the invention disclosed in Japanese Patent Application Laid-Open No. H09-113930 also has problems. According to the method, the conductive layer for repairing line disconnection is additionally provided on the pixel electrode 12. Therefore, the manufacturing process requires an additional step of forming the conductive layer. Also, the conductive layer is placed on the pixel electrode applying an electric field to liquid crystals. Therefore, after the repair, an electric potential of a source signal, gate signal, or common signal is directly applied to the liquid crystals through the conductive layer. It causes noise to have an adverse effect on operations of the liquid crystals, resulting in poor display quality.
As described above, the conventional method for repairing line disconnection has the problems of a reduced rate of successful repair and deteriorated display quality caused by an expected short-circuit and noise.